based on past experience in our environment, it doesn't look up a new GC automatically. Most, of our domestic DCs are GCs as well...I know which ones are which, just don't know how to force the MAPI profile to use a different one other than a reboot (maybe a full stop and start of BES services would work the same way?). They'll be down for 4+ hours each, so a reboot is better than just waiting out the DC outage even though we rarely reboot our BES servers any more.
BES 4.1.7, Exchange 2007 SP2, SQL 2005, Windows 2003 Server on an ESX VMWare. T-Mobile = our preferred carrier.